Method for production of firmly adhering galvanic coatings on aluminum and aluminum alloys



' Patented Apr. 7, 1936 PATENT OFFICE METHOD FOR PRODUCTION OF FIBMLYAD- HERING. GALVANIO COATINGS ON ALU- LIINUM AND ALUMINUM ALLOYSJohannes Fischer, Berlin-Charlottenburg, Germany, assignor to Siemens 4;Halske,-Akti gescllschatt, Siemensstadt, near en- Berlin, Germany, acorporation of Germany No Drawing, Application February 1, 1934, SerialNo. 709,349. In Germany February 15, 1933 2 Claims. (01. 204-3) Thisinvention relates to a method for production of firmly adhering galvaniccoatings on aluminum and aluminum alloys.

' According to prior proposals one produces 5 firmly adhering coatingson aluminum and aluminum alloys by first producing anoxide layer byanodic treatment of the metals preferably in baths. containing chromicacid, then detaching partially the so formed oxide layer by cathodictreatment in alkaline baths containing salts ot heavy metals, e. g.copper and so simultaneously forming an intermediate copper layer ofinterrupted structure, finally plating the so prepared body. It is alsoknown to use alternating current and an alkaline bath for the productionor the artificial oxide layer.

It has now been discovered that it is possible to get metal coatings ofparticularly firm adherence on aluminum and its alloys by treating thebodies in a phosphoric acid solution with direct or alternating currentto produce an oxide film on their surfaces.

The concentration of the-phosphoric acid solution may vary within widelimits, the use 0! concentrations between one andten percent beingadvantageous. It is further advantageous, but .not necessary to keep thetemperature or the bath constant at forty to fiity degrees centigrade.Lower temperatures are used ii the aluminum alloys" contain greaterpercentages of heavy metals.

After treating the bodies in the phosphoric acid bath one continues theprocess according to the method hereinbeiore described, i. e. one

makes the oxidized articles cathodes in an alkaline bath containingcopper or brass whereby a part of the oxide layer disappears and a smallquantity of copper or brass is simultaneously precipitated, and finallyplates the body so prepared.

' In its broad aspects my invention comprises producing an oxide layeron an article of aluminum or of an aluminum alloy by electricaltreatment thereof in an aqueous phosphoric acid bath, the concentrationof the acid, temperature of the bath, duration of the treatment andother conditions being so correlated and controlled as to form anadherent oxide layer, which layer is sufliciently porous to receive anadherent metallic flash coating in an alkaline plating bath,

thereupon flash-coatingsaid article over the resulting oxide layer'in analkaline electroplating, bath, and then finish-plating 881d article. The

oxide layer produced inmy process should corsubstantiallyin character tothat produced by the specific examplewhich follows.

In an illustrative example of my process an aluminum article was putinto a. three percent bath of phosphoric acid at about forty degreescentigrade. This article was made an anode.

The initial current reached a value of about 5 amperes,. This currentdecreased within a few seconds to about twenty amperes, the voltageincreasing to ninety or hundred volts simultaneously. The article havingbeen so oxidized during five minutes was then carefully rinsed and input into an alkaline bath containing copper and zinc salts. It wastherefiash-coated with brass as an intermediate metal by being made a'cathode at a voltage of five volts. Sufficient brass was precipitatedafter five minutes treating. After 15 a above:

(1) In case of insuilicient rinsing the plies-- phoric acid does notimpair the alkaline bath as does thechromic acid. Chromic acidinterieres with the second stage of the coating process as it 30aii'ects generally all galvanicprocesses.

(2) The handling of chromic acid is avoided with its attendant risks tothe health oi the workmen.

(3) A'great number of different-alloys, for ex- 35 ample, magnesiumcontaining alloys, may be oxidized in phosphoric acid baths which isoften impossible it chromic acid is used aselectrolyte or ii theoxidation process is conducted with al- 0 ternating current in asolution or sodium carbonate.

(4) The process may be operated with direct alternating current in thesame bath at will.

5) There is no strict rule. for the duration of the oxidation period.Using chromic acid the oxide layer after a fixed period will become sothick thatitcannotbedetachedbythecathodic treatmentinthealkalinebath.Umphosphoric acid the time-of treatingis much lea critical. 60

rorexampleitispcssibletoallowthearflelesto stay in the phosphoric acidbath during a whole hour or more without deterioration of the pastprecipitated-oxide This commercial practice because it simplifies thecon trol of the process. 1

Further it was found that the ei'ilcacy of the oxide layer is dependentupon its porosity. If its thickness becomes so great that current cannotpass at all, the oxide layer--as explained hereinbefore-cannot furtherform a suitable base for the superimposed copper or brasscoating,-because it is very diflicult to partially detach such oxidelayers in the alkaline bath.

Three factors must be carefully controlledin the production of porouslayers:

(a) duration of oxidation,

(b) temperature and (0) concentration of the electrolyte.

The importance of the duration has been mentioned hereinbefore. Thetemperature of the electrolyte has also great influence on the characterofthe layer, which becomes more porous at higher temperatures whilebeing difllcult to produce in such porous form at lower temperatures.Therefore the duration of oxidation also may be varied within widerlimits at higher temperatures than at lower ones.

Similarly porous layers are more easily attained at higher than at lowerconcentrations of the electrolyte.

It is known to form oxide layers on aluminum electrodes for the purposeof polarization. It is furthermore well known to treat aluminum bodieswith direct or alternating current in soda or barium peroxide solutionscontaining small quantitles of phosphoric acid for producing insulatinglayers on aluminum. But these known methods are comparable only with thefirst stage of the process according to this invention which ischaracterized by the unity of its three stages and serves for quitediii'erent purposes.

What I claim is:

1. In the plating of firmly adherent metallic coatings on aluminum andaluminum alloys, the process which comprises producing an oxide layer onsuch an article by electrical treatment thereof in aqueous phosphoricacid bath, the concentration of the acid, temperature of the bath,duration of the treatment and other conditions being so correlated andcontrolled as to form an adherent oxide layer, which layer issufflciently porous to receive an adherent metallic flash coating in analkaline plating bath, thereupon flash-coating said article over theresulting oxide .layer in an alkaline electroplating bath, and thenfinish-plating said article.

2. The process of claim l.wherein the conditions of the treatment in thephosphoric acid bath are so correlated as to correspond substantially toa treatment for about five minutes in an aqueous bath of phosphoric acidof about 3' per cent concentration, maintained at a temperature rangingfrom about 40 to C., and with a voltage increasing during treatment to avalue of from about to volts.

JOHANNES FISCHER.

